How to lock texture tile size while painting?
I found a partial answer. If you want to keep the tile size same while texture painting, you should paint textures only inside of the UV editor.


Unwrap your mesh.
Change your view to the "UV/Image editor".
Select "View" and change it to the "Paint".
Then go back to the "3D View" and select the "Texture Paint".
Go back to the "UV/Image editor" and now you could paint textures directly to the texture atlas.


You could control the tile size by adjusting the Brush Radius.